Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
L1/2 Control Signalling
In order to inform the scheduled users about their allocation status, transport format and other data related information (e.g. HARQ) L1/L2 control signaling needs to be transmitted on the downlink along with the data. The control signaling needs to be multiplexed with the downlink data in a sub frame (assuming that the user allocation can change from sub frame to sub frame). Here, it should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length may be a multiple of the sub frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/2 control signaling needs only be transmitted once per TTI. The L1/L2 control signalling is transmitted on the Physical Downlink Control Channel (PDCCH). It should be noted that assignments for uplink data transmissions, UL grants, are also transmitted on the PDCCH.
Generally, the information sent on the L1/L2 control signaling may be separated into the following two categories:                Shared Control Information (SCI) carrying Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                    User identity, indicating the user which is allocated            RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.            Optional: Duration of assignment, if an assignment over multiple sub frames (or TTIs) is possible                        Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).        Dedicated Control Information (DCI) carrying Cat 2/3 information        The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                    Cat 2: Modulation scheme, transport block (payload) size (or coding rate), MIMO related information, etc.            Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence number                        
In the following the detailed L1/L2 control signalling information signalled for DL allocation respectively uplink assignments is described in the following:                Downlink Data Transmission:        Along with the downlink packet data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH). This L1/L2 control signaling typically contains information on:                    The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.            The transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de rate matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.            Hybrid ARQ (HARQ) information:                            Process number: Allows the UE to identify the hybrid ARQ process on which the data is mapped                Sequence number or new data indicator: Allows the UE to identify if the transmission is a new packet or a retransmitted packet                Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version is used (required for de-rate matching) and/or which modulation constellation version is used (required for demodulation)                                    UE Identity (UE ID): Tells for which UE the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.                        Uplink Data Transmission:        To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the UE about the transmission details. This L1/L2 control signaling typically contains information on:                    The physical resource(s) on which the UE should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).            The transport Format, the UE should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.            Hybrid ARQ information:                            Process number: Tells the UE from which hybrid ARQ process it should pick the data                Sequence number or new data indicator: Tells the UE to transmit a new packet or to retransmit a packet                Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version to use (required for rate matching) and/or which modulation constellation version to use (required for modulation)                                    UE Identity (UE ID): Tells which UE should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.                        
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:                HARQ process number may not be needed in case of a synchronous HARQ protocol        A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre defined.        Power control information may be additionally included in the control signaling        MIMO related control information, such as e.g. precoding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included        
For uplink resource assignments (PUSCH) signalled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The TF respectively MCS field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating RVs 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits. Further detailed information on the control information for uplink resource allocation on PUSCH can be found in TS36.212 section 5.3.3 and TS36.213 section 8.6.
For downlink assignments (PDSCH) signalled on PDCCH in LTE the Redundancy Version (RV) is signalled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signalled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signalled. Further detailed information on the control information for uplink resource allocation on PUSCH can be found in TS36.212 section 5.3.3 and TS36.213 section 7.1.7
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE (Release 8) each sub-frame is divided into two downlink slots as shown in FIG. 3, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0 or 9.0.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the Rel. 8/9 numerology. It is possible to configure a user equipment to aggregate a different number of component carriers originating from the same eNodeB and of possibly different bandwidths in the uplink and the downlink:                The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the user equipment;        The number of uplink component carriers that can be configured depends on the uplink aggregation capability of the user equipment;        It is not possible to configure a user equipment with more uplink component carriers than downlink component carriers;        In typical TDD deployments, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same.        
Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of Rel. 8/9 and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO—Single User Multiple Input Multiple Output—for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier. The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively.
When carrier aggregation is configured, the user equipment only has one RRC connection with the network. At RRC connection establishment/re-establishment, one serving cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected mode. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC), while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
Depending on the user equipment's capabilities, Secondary Cells (SCells) can be configured to form a set of serving cells, together with the PCell. Therefore, the configured set of serving cells for a user equipment always consists of one PCell and one or more SCells. The characteristics of the downlink and uplink PCell and SCells are                The uplink PCell is used for transmission of Layer 1 uplink control information (PUCCH)        Unlike SCells, the downlink PCell cannot be de-activated        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCells and no SCell can be configured for usage of uplink resources only)        From a UE viewpoint, each uplink resource only belongs to one serving cell        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure)        The number of serving cells that can be configured depends on the aggregation capability of the UE        Non-access stratum information is taken from the downlink PCell.        
The reconfiguration, addition and removal of SCells can be performed by RRC. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage with the target PCell. When adding a new SCell, dedicated RRC signaling is used for sending all required system information of the SCell, i.e. while in connected mode, user equipments need not acquire broadcast system information directly from the SCells.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously, but at most one random access procedure should be ongoing at any time. Cross-carrier scheduling allows the Physical Downlink Control Channel (PDCCH) of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field (CIF) is introduced in the respective Downlink Control Information (DCI) formats. A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carriers does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Activation/Deactivation of SCells
To enable reasonable UE battery consumption when CA is configured, an activation/deactivation mechanism of SCells is supported (i.e. activation/deactivation does not apply to PCell). When an SCell is deactivated, the UE does not need to receive the corresponding PDCCH or PDSCH, cannot transmit in the corresponding uplink, nor is it required to perform CQI measurements. Conversely, when an SCell is active, the UE shall receive PDSCH and PDCCH (if the UE is configured to monitor PDCCH from this SCell), and is expected to be able to perform CQI measurements.
The activation/deactivation mechanism is based on the combination of a MAC control element and deactivation timers. The MAC control element carries a bitmap for the activation and deactivation of SCells: a bit set to 1 denotes activation of the corresponding SCell, while a bit set to 0 denotes deactivation. With the bitmap, SCells can be activated and deactivated individually, and a single activation/deactivation command can activate/deactivate a subset of the SCells. The corresponding activation/deactivation MAC CE is shown in FIG. 20. It should be noted, that even though there is a maximum of 4 Scells a UE can aggregate, the MAC CE contains 7 entries, each of them corresponding to an SCell configured with SCellIndex i.
One deactivation timer is maintained per SCell but one common value is configured per UE by RRC. At reconfiguration without mobility control information:                SCells added to the set of serving cells are initially “deactivated”;        SCells which remain in the set of serving cells (either unchanged or reconfigured) do not change their activation status (“activated” or “deactivated”).        
At reconfiguration with mobility control information (i.e. handover):                SCells are “deactivated”.Uplink Access Scheme for LTE        
For Uplink transmission, power-efficient user-terminal transmission is necessary to maximize coverage. Single-carrier transmission combined with FDMA with dynamic bandwidth allocation has been chosen as the evolved UTRA uplink transmission scheme. The main reason for the preference for single-carrier transmission is the lower peak-to-average power ratio (PAPR), compared to multi-carrier signals (OFDMA), and the corresponding improved power-amplifier efficiency and assumed improved coverage (higher data rates for a given terminal peak power). During each time interval, Node B assigns users a unique time/frequency resource for transmitting user data thereby ensuring intra-cell orthogonality. An orthogonal access in the uplink promises increased spectral efficiency by eliminating intra-cell interference. Interference due to multipath propagation is handled at the base station (Node B), aided by insertion of a cyclic prefix in the transmitted signal.
The basic physical resource used for data transmission consists of a frequency resource of size BWgrant during one time interval, e.g. a sub-frame of 0.5 ms, onto which coded information bits are mapped. It should be noted that a sub-frame, also referred to as transmission time interval (TTI), is the smallest time interval for user data transmission. It is however possible to assign a frequency resource BWgrant over a longer time period than one TTI to a user by concatenation of sub-frames.
Uplink Scheduling Scheme for LTE
The uplink scheme allows for both scheduled access, i.e. controlled by eNB, and contention-based access.
In case of scheduled access the UE is allocated a certain frequency resource for a certain time (i.e. a time/frequency resource) for uplink data transmission. However, some time/frequency resources can be allocated for contention-based access. Within these time/frequency resources, UEs can transmit without first being scheduled. One scenario where UE is making a contention-based access is for example the random access, i.e. when UE is performing initial access to a cell or for requesting uplink resources.
For the scheduled access Node B scheduler assigns a user a unique frequency/time resource for uplink data transmission. More specifically the scheduler determines                which UE(s) that is (are) allowed to transmit,        which physical channel resources (frequency),        Transport format (Modulation Coding Scheme (MCS)) to be used by the mobile terminal for transmission        
The allocation information is signalled to the UE via a scheduling grant, sent on the L1/L2 control channel. For simplicity reasons this channel is called uplink grant channel in the following. A scheduling grant message contains at least information which part of the frequency band the UE is allowed to use, the validity period of the grant, and the transport format the UE has to use for the upcoming uplink transmission. The shortest validity period is one sub-frame. Additional information may also be included in the grant message, depending on the selected scheme. Only “per UE” grants are used to grant the right to transmit on the UL-SCH (i.e. there are no “per UE per RB” grants). Therefore the UE needs to distribute the allocated resources among the radio bearers according to some rules, which will be explained in detail in the next section. Unlike in HSUPA there is no UE based transport format selection. The eNB decides the transport format based on some information, e.g. reported scheduling information and QoS info, and UE has to follow the selected transport format. In HSUPA Node B assigns the maximum uplink resource and UE selects accordingly the actual transport format for the data transmissions.
Since the scheduling of radio resources is the most important function in a shared channel access network for determining Quality of service, there are a number of requirements that should be fulfilled by the UL scheduling scheme for LTE in order to allow for an efficient QoS management.                Starvation of low priority services should be avoided        Clear QoS differentiation for radio bearers/services should be supported by the scheduling scheme        The UL reporting should allow fine granular buffer reports (e.g. per radio bearer or per radio bearer group) in order to allow the eNB scheduler to identify for which Radio Bearer/service data is to be sent.        It should be possible to make clear QoS differentiation between services of different users        It should be possible to provide a minimum bit rate per radio bearer        
As can be seen from above list one essential aspect of the LTE scheduling scheme is to provide mechanisms with which the operator can control the partitioning of its aggregated cell capacity between the radio bearers of the different QoS classes. The QoS class of a radio bearer is identified by the QoS profile of the corresponding SAE bearer signalled from AGW to eNB as described before. An operator can then allocate a certain amount of its aggregated cell capacity to the aggregated traffic associated with radio bearers of a certain QoS class. The main goal of employing this class-based approach is to be able to differentiate the treatment of packets depending on the QoS class they belong to.
Medium Access Control (MAC) and MAC Control Elements
The MAC layer is the lowest sub-layer in the Layer 2 architecture of the LTE radio protocol stack (see 3GPP TS 36.321, “Medium Access Control (MAC) protocol specification”, version 8.7.0, in particular sections 4.2, 4.3, 5.4.3 and 6, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The connection to the physical layer below is through transport channels, and the connection to the RLC layer above is through logical channels. The MAC layer performs multiplexing and demultiplexing between logical channels and transport channels. The MAC layer in the transmitting side (in the following examples the user equipment) constructs MAC PDUs, also referred to as transport blocks, from MAC SDUs received through logical channels, and the MAC layer in the receiving side recovers MAC SDUs from MAC PDUs received through transport channels.
In the multiplexing and demultiplexing entity, data from several logical channels can be (de)multiplexed into/from one transport channel. The multiplexing entity generates MAC PDUs from MAC SDUs when radio resources are available for a new transmission. This process includes prioritizing the data from the logical channels to decide how much data and from which logical channel(s) should be included in each MAC PDU. Please note that the process of generating MAC PDUs in the user equipment is also referred to a logical channel prioritization (LCP) in the 3GPP terminology.
The demultiplexing entity reassembles the MAC SDUs from MAC PDUs and distributes them to the appropriate RLC entities. In addition, for peer-to-peer communication between the MAC layers, control messages called ‘MAC Control Elements’ can be included in the MAC PDU.
A MAC PDU primarily consists of the MAC header and the MAC payload (see 3GPP TS 36.321, section 6). The MAC header is further composed of MAC sub-headers, while the MAC payload is composed of MAC Control Elements, MAC SDUs and padding. Each MAC sub-header consists of a Logical Channel ID (LCID) and a Length (L) field. The LCID indicates whether the corresponding part of the MAC payload is a MAC Control Element, and if not, to which logical channel the related MAC SDU belongs. The L field indicates the size of the related MAC SDU or MAC Control Element. As already mentioned above, MAC
Control Elements are used for MAC-level peer-to-peer signaling, including delivery of BSR information and reports of the UE's available power in the uplink, and in the downlink DRX commands and timing advance commands. For each type of MAC Control Element, one special LCID is allocated. An example for a MAC PDU is shown in FIG. 21.
Power Control
Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment. In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells. The role of the Power Control (PC) becomes decisive to provide the required SINR (Signal-to-Interference plus Noise Ratio) while controlling at the same time the interference caused to neighboring cells. The idea of classic PC schemes in uplink is that all users are received with the same SINR, which is known as full compensation. As an alternative, the 3GPP has adopted the use of Fractional Power Control (FPC) for LTE Rel. 8/9. This new functionality makes users with a higher path-loss operate at a lower SINR requirement so that they will more likely generate less interference to neighboring cells.
The power control scheme provided in LTE Rel. 8/9 employs a combination of open-loop and closed-loop control. A mode of operation involves setting a coarse operating point for the transmission power density spectrum by open-loop means based on path-loss estimation. Faster operation can then be applied around the open-loop operating point by closed-loop power control. This controls interference and fine-tunes the power settings to suit the channel conditions including fast fading.
With this combination of mechanisms, the power control scheme in LTE Rel. 8/9 provides support for more than one mode of operation. It can be seen as a toolkit for different power control strategies depending on the deployment scenario, the system load and operator preference.
The detailed power control formulae are specified in LTE Rel. 8/9 for the Physical Uplink Shared Channel (PUSCH), Physical Uplink Control Channel (PUCCH) and the Sounding Reference Signals (SRS) in section 5.1 in 3GPP TS 36.213, “Physical layer procedures”, version 8.8.0, available at http://www.3gpp.org and incorporated herein by reference. The formula for each of these uplink signals follows the same basic principles; in all cases they can be considered as a summation of two main terms: a basic open-loop operating point derived from static or semi-static parameters signaled by the eNodeB, and a dynamic offset updated from sub-frame to sub-frame.
The basic open-loop operating point for the transmit power per resource block depends on a number of factors including the inter-cell interference and cell load. It can be further broken down into two components, a semi-static base level P0, further comprised of a common power level for all user equipments in the cell (measured in dBm) and a UE-specific offset, and an open-loop path-loss compensation component. The dynamic offset part of the power per resource block can also be further broken down into two components, a component dependent on the MCS and explicit Transmitter Power Control (TPC) commands.
The MCS-dependent component (referred to in the LTE specifications as ΔTF, where TF stands for “Transport Format”) allows the transmitted power per resource block to be adapted according to the transmitted information data rate.
The other component of the dynamic offset is the UE-specific TPC commands. These can operate in two different modes: accumulative TPC commands (available for PUSCH, PUCCH and SRS) and absolute TPC commands (available for PUSCH only). For the PUSCH, the switch between these two modes is configured semi-statically for each UE by RRC signaling—i.e. the mode cannot be changed dynamically. With the accumulative TPC commands, each TPC command signals a power step relative to the previous level. Uplink transmitter power control in a mobile communication system serves the purpose of balancing the need for sufficient transmitter energy per bit to achieve the required QoS against the need to minimize interference to other users of the system and to maximize the battery life of the user equipment.
In achieving this, the uplink power control has to adapt to the characteristics of the radio propagation channel, including path loss, shadowing and fast fading, as well as overcoming interference from other users within the same cell and neighboring cells.
The setting of the UE Transmit power PPUSCH [dBm] for the PUSCH transmission in reference sub-frame i is defined by (see section 5.1.1.1 of 3GPP TS 36.213):PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}   (Equation 1)                PCMAX is the maximum UE transmit power chosen by the UE in the given range (see below);        MPUSCH is the number of allocated physical resource blocks (PRBs). The more PRBs are allocated, the more uplink transmit power is allocated.        P0_PUSCH (j) indicates the base transmission power signaled by RRC. For semi-persistent scheduling (SPS) and dynamic scheduling this is the sum of a cell specific nominal component PO_NOMINAL_PUSCH(j)ε[−126, . . . , 24] and a UE specific component PO_UE_PUSCH (j)ε[127, . . . , −96]. For RACH message 3: Offset from preamble transmission power        α denotes a cell-specific parameter (that is broadcast on system information). This parameter indicates how much path-loss PL is compensated. α=1 means the received signal level at eNodeB is same regardless of the user equipment's position, i.e. near cell edge or at centre. If the path-loss is fully compensated, degradation to the cell-edge data rate is avoided. For SPS and dynamic scheduling αε{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}, and for the case of RACH Message 3, α(j)=1.        PL is the UE path-loss derived at the user equipments based on Reference Signal Received Power (RSRP) measurement and signaled Reference Signal (RS) transmission power. PL can be defined asPL=reference signal power−higher layer filtered RSRP.        ΔTF is a modulation and coding scheme (transport format) dependent power offset. It thus allows the transmitted power per resource block to be adapted according to the transmitted information data rate.        f(i) is a function of the closed loop power control commands signaled from the eNodeB to the UE. f( ) represents accumulation in case of accumulative TPC commands. Whether closed loop commands are accumulative (each TPC command signals a power step relative to the previous level) or absolute (each TCP command is independent of the sequence of previous TPC commands) is configured by higher layers. For the accumulative TPC commands two sets of power step values are provided: (−1,1) dB for DCI format 3A and (−1,0+1,+3)dB for DCI format 3. The set of values which can be signaled by absolute TPC commands is (−4,−1,1,4) dB indicated by DCI format 3.Power Headroom Reporting        
In order to assist the eNodeB to schedule the uplink transmission resources to different user equipments in an appropriate way, it is important that the user equipment can report its available power headroom to eNodeB.
The eNodeB can use the power headroom reports to determine how much more uplink bandwidth per sub-frame a user equipment is capable of using. This helps to avoid allocating uplink transmission resources to user equipments which are unable to use them in order to avoid a waste of resources.
The range of the power headroom report is from +40 to −23 dB (see 3GPP TS 36.133, “Requirements for support of radio resource management”, version 8.7.0, section 9.1.8.4, available at http//www.3gpp.org and incorporated in its entirety herein by reference). The negative part of the range enables the user equipment to signal to the eNodeB the extent to which it has received an UL grant which would require more transmission power than the UE has available. This would enable the eNodeB to reduce the size of a subsequent grant, thus freeing up transmission resources to allocate to other UEs.
A power headroom report can only be sent in sub-frames in which a UE has an UL transmission grant. The report relates to the sub-frame in which it is sent. The headroom report is therefore a prediction rather than a direct measurement; the UE cannot directly measure its actual transmission power headroom for the sub-frame in which the report is to be transmitted. It therefore relies on reasonably accurate calibration of the UE's power amplifier output.
A number of criteria are defined to trigger a power headroom report. These include:                A significant change in estimated path loss since the last power headroom report        More than a configured time has elapsed since the previous power headroom report        More than a configured number of closed-loop TPC commands have been implemented by the UE        
The eNodeB can configure parameters to control each of these triggers depending on the system loading and the requirements of its scheduling algorithm. To be more specific, RRC controls power headroom reporting by configuring the two timers periodicPHR-Timer and prohibitPHR-Timer, and by signalling dl-PathlossChange which sets the change in measured downlink pathloss to trigger a power headroom report.
The power headroom report is send as a MAC Control Element. It consists of a single octet where the two highest bits are reserved and the six lowest bits represent the 64 dB values mentioned above in 1 dB steps. The structure of the MAC Control Element is shown in FIG. 40.
The UE power headroom PH [dB] valid for sub-frame i is defined by (see section 5.1.1.2 of 3GPP TS 36.213):PH(i)=PCMAX−{10·log10(MPUSCH(i))+PO_PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}  (Equation 2)
The power headroom is rounded to the closest value in the range [40; −23] dB with steps of 1 dB. PCMAX is the total maximum UE transmit power (or total maximum transmit power of the user equipment) and is a value chosen by user equipment in the given range of PCMAX_L and PCMAX_H based on the following constraints:                PCMAX_L≦PCMAX≦PCMAX_H         PCMAX_L=min(PEMAX−ΔTC, PPowerClass−MPR−AMPR−ΔTC         PCMAX_H=min(PEMAX, PPowerClass)        
PEMAX is the value signaled by the network and ΔTC, MPR and AMPR (also denoted A-MPR—Additional Maximum Power Reduction) are specified in 3GPP TS 36.101, “Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception”, version 8.7.0, section 6.2 available at http//www.3gpp.org and incorporated herein by reference.
MPR is a power reduction value, the so-called Maximum Power Reduction, used to control the Adjacent Channel Leakage Power Ratio (ACLR) associated with the various modulation schemes and the transmission bandwidth. An adjacent channel may be for example either another Evolved Universal Terrestrial Radio Access (E-UTRA) channel or an UTRA channel. The maximum allowed power reduction (MPR) is also defined in 3GPP TS 36.101. It is different depending on channel bandwidth and modulation scheme. The user equipment's reduction may be less than this maximum allowed power reduction (MPR) value. 3GPP specifies a MPR test which verifies that the maximum transmit power of a user equipment is greater than or equal to the nominal total maximum transmit power minus the MPR while still complying with the ACLR requirements
The following Table 1 shows the Maximum Power Reduction for UE Power Class 3.
TABLE 1maximum power reduction for UE power class 3Channel bandwidthTransmission bandwidth configuration(resource blocks)Modu-1.435101520MPRlationMHzMHzMHzMHzMHzMHz(dB)QPSK>5>4>8>12>16>18≦116 QAM≦5≦4≦8≦12≦16≦18≦116 QAM>5>4>8>12>16>18≦2
For instance, in case of an allocation for a channel bandwidth of 10 MHz, when allocating more than 12 resource blocks and using QPSK modulation, the MPR applied by the user equipment should be smaller than or equal to 1 dB. The actual MPR applied by the user equipment depends on the implementation of the UE and is thus unknown to the eNB.
As indicated above, AMPR is the Additional Maximum Power Reduction. It is band specific and is applied when configured by the network. As can be seen from the explanations above, PCMAX is UE-implementation specific and hence not known by the eNodeB.
FIG. 41 shows exemplary scenarios for a UE transmission power status and corresponding power headroom. On the left hand side of FIG. 41, the user equipment is not power limited (positive PHR), whereas on the right hand side of FIG. 41 a negative power headroom is implying a power limitation of the user equipment. Please note that the PCMAX_L≦PCMAX≦min(PEMAX, PPowerClass) wherein the lower boundary PCMAX_L is typically mainly dependent on the maximum power reduction MPR and the additional maximum power reduction AMPR, i.e. PCMAX_L≅PPowerClass−MPR−AMPR
Uplink Power Control for Carrier Aggregation
One main point of UL Power control for LTE-Advance is that a component carrier specific UL power control is supported, i.e. there will be one independent power control loop for each UL component carrier configured for the UE. Furthermore power headroom is reported per component carrier.
In Rel-10 within the scope of carrier aggregation there are two maximum power limits, a maximum total UE transmit power and a CC-specific maximum transmit power. RAN1 agreed at the RAN1#60bis meeting that a power headroom report, which is reported per CC, accounts for the maximum power reduction (MPR). In other words the power reduction applied by the UE is taken into account in the CC-specific maximum transmission power PCMAX,c (c denotes the component carrier).
Different to Rel-8/9, for LTE-A the UE has also to cope with simultaneous PUSCH-PUCCH transmission, multi-cluster scheduling and simultaneous transmission on multiple CCs, which requires larger MPR values and also causes a larger variation of the applied MPR values compared to Rel-8/9.
It should be noted that the eNB does not have knowledge of the power reduction applied by the UE on each CC, since the actual power reduction depends on the type of allocation, the standardized MPR value and also on the UE implementation. Therefore eNB doesn't know the CC-specific maximum transmission power relative to which the UE calculates the PHR. In Rel-8/9 for example UE's maximum transmit power Pcmax can be within some certain range as described above.PCMAX_L≦PCMAX≦PCMAX_H 
Due to the fact that the power reduction applied by the UE to the maximum transmit power of a CC is not known by eNB it was agreed to introduce in Rel-10 a new power headroom MAC control element, which is also referred to as extended power headroom MAC control element. The main difference to the Rel-8/9 PHR MAC CE format, is that it includes a Rel-8/9 power headroom value for each activated UL CC and is hence of variable size. Furthermore it not only reports the power headroom value for a CC but also the corresponding Pcmax,c (maximum transmit power of CC with the index c) value. In order to account for simultaneous PUSCH-PUCCH transmissions, UE reports for PCell the Rel-8/9 power headroom value which is related to PUSCH only transmissions (referred to type 1 power headroom) and if the UE is configured for simultaneous PUSCH-PUCCH transmission, a further Power headroom value, which considers PUCCH and PUSCH transmissions, also referred to as type 2 power headroom (see FIG. 21). Further details of the extended power headroom MAC Control element can be found in section 6.1.3.6a of TS36.321.
If the total transmit power of the UE, i.e. sum of transmission power on all CCs, would exceed the maximum UE transmit power {circumflex over (P)}CMAX (i), the UE needs to scale down uplink transmission power on PUSCH/PUCCH. There are certain rules for the prioritization of the uplink channels during power scaling defined. Basically control information transmitted on the PUCCH has the highest priority, i.e. PUSCH transmissions are scaled down first before PUCCH transmission power is reduced. This can be also expressed by the following condition which needs to be fulfilled:
            ∑      c                            ⁢                  ⁢                  w        ⁡                  (          i          )                    ·                                    P            ^                                PUSCH            ,            c                          ⁡                  (          i          )                      ≤      (                                        P            ^                    CMAX                ⁡                  (          i          )                    -                                    P            ^                    PUCCH                ⁡                  (          i          )                      )  where {circumflex over (P)}PUCCH(i) is the linear value of PPUCCH(i) (PUCCH transmission power in sub-frame i), {circumflex over (P)}PUSCH,c(i) is the linear value of PPUSCH,c(i) (PUSCH transmission power on carrier c in sub-frame i), {circumflex over (P)}CMAX(i) is the linear value of the UE total configured maximum output power PCMAX in sub-frame i and w(i) is a scaling factor of {circumflex over (P)}PUSCH,c(i) for serving cell c where 0≦w(i)≦1. In case there is no PUCCH transmission in sub-frame i, {circumflex over (P)}PUCCH(i)=0.
For the case that the UE has PUSCH transmission with Uplink control information (UCI) on serving cell j and PUSCH without UCI in any of the remaining serving cells, and the total transmit power of the UE would exceed {circumflex over (P)}CMAX (i), the UE scales {circumflex over (P)}PUSCH,c(i) for the serving cells without UCI in sub-frame i such that the condition
            ∑              c        ≠        j                                  ⁢                  ⁢                  w        ⁡                  (          i          )                    ·                                    P            ^                                PUSCH            ,            c                          ⁡                  (          i          )                      ≤      (                                        P            ^                    CMAX                ⁡                  (          i          )                    -                                    P            ^                                PUSCH            ,            j                          ⁡                  (          i          )                      )  is satisfied where {circumflex over (P)}PUSCH,j(i) is the PUSCH transmit power for the cell with UCI and w(i) is a scaling factor of {circumflex over (P)}PUSCH,c(i) for serving cell c without UCI. In this case, no power scaling is applied to {circumflex over (P)}PUSCH,j(i) unless
            ∑              c        ≠        j                                  ⁢                  ⁢                            w          ⁡                      (            i            )                          ·                              P            ^                                PUSCH            ,            c                              ⁢              (        i        )              =  0and the total transmit power of the UE still would exceed {circumflex over (P)}CMAX (i). Note that w(i) values are the same across serving cells when w(i)>0 but for certain serving cells w(i) may be zero.
If the UE has simultaneous PUCCH and PUSCH transmission with UCI on serving cell j and PUSCH transmission without UCI in any of the remaining serving cells, and the total transmit power of the UE would exceed {circumflex over (P)}CMAX (i), the UE obtains {circumflex over (P)}PUSCH,c(i) according to
                              P          ^                          PUSCH          ,          j                    ⁡              (        i        )              =          min      ⁡              (                                                            P                ^                                            PUSCH                ,                j                                      ⁡                          (              i              )                                ,                      (                                                                                P                    ^                                    CMAX                                ⁡                                  (                  i                  )                                            -                                                                    P                    ^                                                        PUSCH                    ,                    j                                                  ⁡                                  (                  i                  )                                                      )                          )              and                    ∑                  c          ≠          j                                              ⁢                          ⁢                                    w            ⁡                          (              i              )                                ·                                    P              ^                                      PUSCH              ,              c                                      ⁢                  (          i          )                      ≤          (                                                  P              ^                        CMAX                    ⁡                      (            i            )                          -                                            P              ^                        PUCCH                    ⁡                      (            i            )                          -                                            P              ^                                      PUSCH              ,              j                                ⁡                      (            i            )                              )      Timing Advance
For the uplink transmission scheme of 3GPP LTE single-carrier frequency division multiple access (SC-FDMA) was chosen to achieve an orthogonal multiple-access in time and frequency between the different user equipments transmitting in the uplink.
Uplink orthogonality is maintained by ensuring that the transmissions from different user equipments in a cell are time-aligned at the receiver of the eNodeB. This avoids intra-cell interference occurring, both between user equipments assigned to transmit in consecutive sub-frames and between user equipments transmitting on adjacent subcarriers. Time alignment of the uplink transmissions is achieved by applying a timing advance at the user equipment's transmitter, relative to the received downlink timing as exemplified in FIG. 9c. The main role of this is to counteract differing propagation delays between different user equipments.
FIG. 9a illustrates the misalignment of the uplink transmissions from two mobile terminals in case no uplink timing alignments is performed, such that the eNodeB receives the respective uplink transmissions from the two mobile terminals at different timings.
FIG. 9b in contrast thereto illustrates synchronized uplink transmissions for two mobile terminals. The uplink timing alignment is performed by each mobile terminal and applied to the uplink transmissions such that at the eNodeB the uplink transmissions from the two mobile terminals arrive at substantially the same timing.
Initial Timing Advance Procedure
When user equipment is synchronized to the downlink transmissions received from eNodeB, the initial timing advance is set by means of the random access procedure as described below. The user equipment transmits a random access preamble based on which the eNodeB can estimate the uplink timing. The eNodeB responds with an 11-bit initial timing advance command contained within the Random Access Response (RAR) message. This allows the timing advance to be configured by the eNodeB with a granularity of 0.52 μs from 0 up to a maximum of 0.67 ms.
Additional information on the control of the uplink timing and timing advance on 3GPP LTE (Release 8/9) can be found in chapter 20.2 of Stefania Sesia, Issam Toufik and Matthew Baker, “LTE—The UMTS Long Term Evolution: From Theory to Practice”, John Wiley & Sons, Ltd. 2009, which is incorporated herein by reference.
Updates of the Timing Advance
Once the timing advance has been first set for each user equipment, the timing advance is updated from time to time to counteract changes in the arrival time of the uplink signals at the eNodeB. In deriving the timing advance update commands, the eNodeB may measure any uplink signal which is useful. The details of the uplink timing measurements at the eNodeB are not specified, but left to the implementation of the eNodeB.
The timing advance update commands are generated at the Medium Access Control (MAC) layer in the eNodeB and transmitted to the user equipment as MAC control elements which may be multiplexed together with data on the Physical Downlink Shared Channel (PDSCH). Like the initial timing advance command in the response to the Random Access Channel (RACH) preamble, the update commands have a granularity of 0.52 μs. The range of the update commands is ±16 μs, allowing a step change in uplink timing equivalent to the length of the extended cyclic prefix. They would typically not be sent more frequently than about every 2 seconds. In practice, fast updates are unlikely to be necessary, as even for a user equipment moving at 500 km/h the change in round-trip path length is not more than 278 m/s, corresponding to a change in round-trip time of 0.93 μs/s.
The eNodeB balances the overhead of sending regular timing update commands to all the UEs in the cell against a UE's ability to transmit quickly when data arrives in its transmit buffer. The eNodeB therefore configures a timer for each user equipment, which the user equipment restarts each time a timing advance update is received. In case the user equipment does not receive another timing advance update before the timer expires, it must then consider that it has lost uplink synchronization (see also section 5.2 of 3GPP TS 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”, version 8.9.0, available at http://www.3gpp.org and incorporated herein by reference).
In such a case, in order to avoid the risk of generating interference to uplink transmissions from other user equipments, the UE is not permitted to make another uplink transmission of any sort and needs to revert to the initial timing alignment procedure in order to restore the uplink timing.
Upon reception of a timing advance command, the user equipment shall adjust its uplink transmission timing for PUCCH/PUSCH/SRS of the primary cell. The timing advance command indicates the change of the uplink timing relative to the current uplink timing as multiples of 16 Ts.
Random Access Procedure
A mobile terminal in LTE can only be scheduled for uplink transmission, if its uplink transmission is time synchronized. Therefore the Random Access (RACH) procedure plays an important role as an interface between non-synchronized mobile terminals (UEs) and the orthogonal transmission of the uplink radio access.
Essentially the Random Access in LTE is used to achieve uplink time synchronization for a user equipment which either has not yet acquired, or has lost, its uplink synchronization. Once a user equipment has achieved uplink synchronization, the eNodeB can schedule uplink transmission resources for it. The following scenarios are therefore relevant for random access:                A user equipment in RRC_CONNECTED state, but not uplink-synchronized, wishing to send new uplink data or control information        A user equipment in RRC_CONNECTED state, but not uplink-synchronized, required to receive downlink data, and therefore to transmit corresponding HARQ feedback, i.e. ACK/NACK, in the uplink. This scenario is also referred to as Downlink data arrival        A user equipment in RRC_CONNECTED state, handing over from its current serving cell to a new target cell; in order to achieve uplink time-synchronization in the target cell, Random Access procedure is performed        A transition from RRC_IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates        Recovering from radio link failure, i.e. RRC connection re-establishment        
There is one more additional case, where user equipment performs random access procedure, even though user equipment is time-synchronized. In this scenario the user equipment uses the random access procedure in order to send a scheduling request, i.e. uplink buffer status report, to its eNodeB, in case it does not have any other uplink resource allocated in which to send the scheduling request, i.e. dedicated scheduling request (D-SR) channel is not configured.
LTE offers two types of random access procedures allowing access to be either contention based, i.e. implying an inherent risk of collision, or contention-free (non-contention based). It should be noted that contention-based random access can be applied for all six scenarios listed above, whereas a non-contention based random access procedure can only be applied for the downlink data arrival and handover scenario.
In the following the contention based random access procedure is being described in more detail with respect to FIG. 7. A detailed description of the random access procedure can be also found in 3GPP 36.321, section 5.1.
FIG. 7 shows the contention based RACH procedure of LTE. This procedure consists of four “steps”. First, the user equipment transmits 701a random access preamble on the Physical Random Access Channel (PRACH) to the eNodeB. The preamble is selected by user equipment from the set of available random access preambles reserved by eNodeB for contention based access. In LTE, there are 64 preambles per cell which can be used for contention-free as well as contention based random access. The set of contention based preambles can be further subdivided into two groups, so that the choice of preamble can carry one bit of information to indicate information relating to the amount of transmission resources needed for the first scheduled transmission, which is referred to as msg3 in TS36.321 (see step 703). The system information broadcasted in the cell contain the information which signatures (preambles) are in each of the two subgroups as well as the meaning of each subgroup. The user equipment randomly selects one preamble from the subgroup corresponding to the size of transmission resource needed for message 3 transmission.
After eNodeB has detected a RACH preamble, it sends 702 a Random Access Response (RAR) message on the PDSCH (Physical Downlink Shared Channel) addressed on the PDCCH with the (Random Access) RA-RNTI identifying the time-frequency slot in which the preamble was detected. If multiple user equipments transmitted the same RACH preamble in the same PRACH resource, which is also referred to as collision, they would receive the same random access response.
The RAR message conveys the detected RACH preamble, a timing alignment command (TA command) for synchronization of subsequent uplink transmissions, an initial uplink resource assignment (grant) for the transmission of the first scheduled transmission (see step 703) and an assignment of a Temporary Cell Radio Network Temporary Identifier (T-CRNTI). This T-CRNTI is used by eNodeB in order to address the mobile(s) whose RACH preamble were detected until RACH procedure is finished, since the “real” identity of the mobile is at this point not yet known by eNodeB.
Furthermore the RAR message can also contain a so-called back-off indicator, which the eNodeB can set to instruct the user equipment to back off for a period of time before retrying a random access attempt. The user equipment monitors the PDCCH for reception of random access response within a given time window, which is configured by the eNodeB. In case user equipment doesn't receive a random access response within the configured time window, it retransmits the preamble at the next PRACH opportunity considering a potentially back off period.
In response to the RAR message received from the eNodeB, the user equipment transmits 703 the first scheduled uplink transmission on the resources assigned by the grant within the random access response. This scheduled uplink transmission conveys the actual random access procedure message like for example RRC connection request, tracking area update or buffer status report. Furthermore it includes either the C-RNTI for user equipments in RRC_CONNECTED mode or the unique 48-bit user equipment identity if the user equipments are in RRC_IDLE mode. In case of a preamble collision having occurred in step 701, i.e. multiple user equipments have sent the same preamble on the same PRACH resource, the colliding user equipments will receive the same T-CRNTI within the random access response and will also collide in the same uplink resources when transmitting 703 their scheduled transmission. This may result in interference that no transmission from a colliding user equipment can be decoded at the eNodeB, and the user equipments will restart the random access procedure after having reached maximum number of retransmission for their scheduled transmission. In case the scheduled transmission from one user equipment is successfully decoded by eNodeB, the contention remains unsolved for the other user equipments.
For resolution of this type of contention, the eNode B sends 704 a contention resolution message addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the 48-bit user equipment identity contained the scheduled transmission of step 703. It supports HARQ. In case of collision followed by a successful decoding of the message sent in step 703, the HARQ feedback (ACK) is only transmitted by the user equipment which detects its own identity, either C-RNTI or unique user equipment ID. Other UEs understand that there was a collision at step 1 and can quickly exit the current RACH procedure and start another one.
FIG. 8 is illustrating the contention-free random access procedure of 3GPP LTE Rel. 8/9. In comparison to the contention based random access procedure, the contention-free random access procedure is simplified. The eNodeB provides 801 the user equipment with the preamble to use for random access so that there is no risk of collisions, i.e. multiple user equipment transmitting the same preamble. Accordingly, the user equipment is sending 802 the preamble which was signaled by eNodeB in the uplink on a PRACH resource. Since the case that multiple UEs are sending the same preamble is avoided for a contention-free random access, no contention resolution is necessary, which in turn implies that step 704 of the contention based procedure shown in FIG. 7 can be omitted. Essentially a contention-free random access procedure is finished after having successfully received the random access response.
When carrier aggregation is configured, the first three steps of the contention-based random access procedure occur on the PCell, while contention resolution (step 704) can be cross-scheduled by the PCell.
The initial preamble transmission power setting is based on an open-loop estimation with full compensation of the path loss. This is designed to ensure that the received power of the preambles is independent of the path-loss.
The eNB may also configure an additional power offset, depending for example on the desired received SINR, the measured uplink interference and noise level in the time-frequency slots allocated to RACH preambles, and possibly on the preamble format. Furthermore, the eNB may configure preamble power ramping so that the transmission for each retransmitted preamble, i.e. in case the PRACH transmission attempt was not successfully, is increased by a fixed step.
The PRACH power is determined by UE through evaluation ofPPRACH=min{PCMAX,c(i),PREAMBLE_RECEIVED_TARGET_POWER+PLC}[dBm],where PCMAX,c(i), is the configured maximum UE transmit power for sub-frame i of the primary cell and PLC is the downlink pathloss estimate calculated in the UE for the primary cell.PREAMBLE_RECEIVED_TARGET_POWER is set to:preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION_COUNTER−1)*powerRampingStep.Channel Quality Feedback Elements
In 3GPP LTE, there exist three basic elements which may or may not be given as feedback for the channel quality:                Modulation and Coding Scheme Indicator (MCSI), which is also referred to as Channel Quality Indicator (CQI) in the 3GPP LTE specifications,        Precoding Matrix Indicator (PMI) and        Rank Indicator (RI)        
The MCSI suggests a modulation and coding scheme that should be employed for downlink transmission to a reporting user equipment, while the PMI points to a precoding matrix/vector that is to be employed for multi-antenna transmission (MIMO) using an assumed transmission matrix rank or a transmission matrix rank that is given by the RI. Details on channel quality reporting and transmission mechanisms are can be found in 3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding (Release 8)”, version 8.7.0, sections 5.2 and 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (Release 8)”, version 8.7.0, section 7.2 (all documents available at http://www.3gpp.org and incorporated herein by reference).
All of these elements are summarized as under the term channel quality feedback herein. Hence, a channel quality feedback can contain any combination of or multiple MCSI, PMI, RI values. Channel quality feedback reports may further contain or consist of metrics such as a channel covariance matrix or elements, channel coefficients, or other suitable metrics as apparent to those skilled in the art.
Channel Quality Feedback in LTE-A (Release 10)
As there is only one component carrier defined in LTE (Release 8), there is no ambiguity at the user equipment on which portion of the system bandwidth CQI reporting is to be done. The CQI request flag (together with the current transmission mode) is unambiguously indicating to the user equipment how to provide CQI feedback to the eNodeB.
With the introduction of carrier aggregation in LTE-A (Release 10) and assuming that the LTE (Release 8) CQI reporting procedures should be reused, there are different possibilities how a CQI request can be interpreted by the user equipment. As shown in FIG. 38, it may be generally assumed that UL-DCI (containing the CQI request) for uplink transmission that is transmitted from a eNodeB or relay node to a user equipment is placed within a single downlink component carrier. A simple rule to handle the CQI request at the user equipment would be that whenever a UL-DCI requests a CQI transmission by the user equipment, same applies to the downlink component carrier where the corresponding UL-DCI is transmitted. I.e. the user equipment would only send aperiodic CQI feedback in a given UL transmission for those downlink component carriers that comprised a UL-DCI requesting a CQI report at the same time.
An alternative handling of UL-DCI comprising a CQI request is shown in FIG. 39. Whenever a UL-DCI requests a CQI transmission by the user equipment, the user equipment applies said request to all downlink component carriers available for downlink transmission to the user equipment.
When downlink transmission can occur on multiple component carriers, an efficient scheduling and link adaptation depends on the availability of accurate and up-to-date CQI. However, in order to make efficient use of the control signaling and CQI transmission resources, it should be possible to control for how many and which component carriers a CQI is to be requested (from the network side) and transmitted (from the terminal side).
According to the first solution discussed above with respect to FIG. 39, in order to request CQI for multiple component carriers the number of component carriers for which CQI is requested is identical to the number of required transmitted UL-DCI messages. In other words, to request CQI for five component carriers it is required to transmit five times more UL-DCI messages than for the case of requesting CQI for just a single component carrier. This solution is therefore not very efficient from a downlink control overhead point of view. According to the second solution above illustrated in FIG. 39, a single uplink DCI message requests CQI for all component carriers. Therefore the downlink control overhead is very small. However, the resulting uplink transmission always requires a large amount of resources to accommodate the transmission of CQI for all component carriers, even though the network knows that it currently requires CQI only for a single selected component carrier. Therefore this is not efficient for the usage of uplink resources, and does not offer any flexibility for the number of requested component carrier CQI.
Timing Advance and Component Carrier Aggregation in the Uplink
In currents specifications of the 3GPP standards the user equipment only maintains one timing advance value and applies this to uplink transmissions on all aggregated component carriers. At the moment, aggregation of cells within the same frequency band is supported, so called intra-frequency carrier aggregation. In particular, uplink timing synchronization is performed for PCell, e.g. by RACH procedure on PCell, and then the user equipment uses the same uplink timing for uplink transmissions on aggregated SCells. A single timing advance for all aggregated uplink component carriers is regarded as sufficient, since current specifications up to 3GPP LTE-A Rel. 10 support only carrier aggregation of carriers from the same frequency band.
However, in the future, e.g. future Release 11, it will be possible to aggregate uplink component carriers from different frequency bands, in which case they can experience different interference and coverage characteristics. Assuming uplink component carriers on widely separated frequency bands, uplink transmissions using the uplink component carriers may be subject to different transmission channel effects. In other words, since transmission channels have to be assumed frequency selective, uplink component carriers on widely separated frequency bands may be differently affected by scattering, multipath propagation and channel fading. Accordingly, the aggregation of uplink component carriers from different frequency bands has to compensate for different propagation delays on the frequency bands.
Furthermore the deployment of technologies like Remote Radio Heads (RRH) as shown for example in FIG. 13 or Frequency Selective Repeaters (FSR) as shown for example in FIG. 14 will also cause different interference and propagation scenarios for the aggregated component carriers. For instance, FIG. 14 illustrates how the FSR relays signals of only frequency f2 relating to the component carrier 2 (CoCa2); signals of frequency f1 are not boosted by the FSR. Consequently, when assuming that the signal strength of the signal from the FSR is greater than the one of the eNodeB (not shown), the component carrier 2 will be received by the user equipment via the frequency-selective repeater, whereas the UE receives the component carrier 1 (CoCa1) directly from the eNodeB. This leads to different propagation delays between the two component carriers.
Therefore, there is a need of introducing more than one timing advance within one user equipment, i.e. separate timing advance may be required for certain component carriers (Serving Cells).
One obvious solution is to perform a RACH procedure also on each of the SCells to achieve uplink synchronization, in a similar way to the PCell. Performing a RACH procedure on a SCell however would result in various disadvantages.
As an immediate consequence, the user equipment would be required to implement the protocol parts of the random access procedure for an aggregated SCell, which affects the Physical Layer, MAC Layer as well as the RRC Layers, hence increasing the UE complexity.
The power control/power allocation procedure is complicated when considering simultaneous transmission of the PRACH and the PUSCH/PUCCH in one sub-frame, i.e. TTI. This might be the case where the uplink of the user equipment is out of synchronization on one component carrier, e.g. SCell, while still being uplink synchronized on another uplink component carrier, e.g. PCell. In order to regain uplink synchronization for the SCell, the user equipment performs a RACH access, e.g. ordered by PDCCH. Consequently, the user equipment transmits a RACH preamble, i.e. performs a PRACH transmission, and in the same TTI the user equipment also transmits PUSCH and/or PUCCH.
Currently, the power control loops for PUSCH/PUCCH and PRACH are totally independent, i.e. PRACH power is not considered when determining PUSCH/PUCCH power, and vice versa. In order to deal with simultaneous PRACH and PUSCH/PUCCH transmissions, changes to the uplink power control algorithm are required. For instance, it would be necessary to consider the PRACH transmission when power scaling needs to be used due to power limitation, since up to now only PUCCH, PUSCH with multiplexed uplink control information (UCI) and PUSCH without UCI are considered for the power limitation case. PUCCH is given the highest priority over PUSCH, and the PUSCH with multiplexed UCI is considered to have a higher priority over PUSCH without UCI.
PUCCH>PUSCH w UCI>PUSCH w/o UCI
The prioritization rules for the power limitation case which are listed in the chapter relating to the Uplink Power Control would have to be extended by PRACH transmissions. However, there is no easy straightforward solution in said respect, since on the one hand uplink control information transmitted on PUCCH or PUSCH have a high priority in order to allow for proper system operation, whereas on the other hand PRACH should be also prioritized in order to ensure a high detection probability at the eNB in order to minimize the delay incurred by the RACH procedure.
In addition to the power control aspects, there is also a further disadvantage regarding the power amplifier that would have to deal with the simultaneous PRACH and PUSCH/PUCCH transmissions. There are certain differences in the uplink timing between the PRACH and the PUSCH/PUCCH, e.g. the timing advance for PRACH is of course 0; Guard Time, GT, which is in the range of 96.88 μs to 715.63 μs. This is depicted in FIG. 22 showing PUSCH transmissions on component carrier 0, and a corresponding PRACH transmission on component carrier 1. As apparent, the PRACH transmission is not time aligned with the PUSCH transmissions as regards the sub-frame.
Due to the Guard Time, there are power fluctuations within one sub-frame, which are undesirable. These power transients will add extra complexity to the implementation of the user equipment; in other words, the maximum power reduction needs to also change during one sub-frame in order to fulfill the EMC requirements.
With the deployment of technologies like Remote Radio Heads (RRHs) or Frequency Selective Repeaters (FSRs), different propagation paths may introduce different propagation delays into the communication between an eNodeB and a user equipment UE. Assuming cell aggregation of different frequency bands, transmissions and/or receptions by the user equipment to/from an eNodeB may be affected by different propagation delays due to different locations of the eNodeB and RRH or FSR and/or frequency selective channel effects as has been explained with respect to FIGS. 13 and 14.
Specifically, the different propagation delays and/or frequency selective channel effects do not only adversely affect uplink transmissions to an eNodeB via plural uplink serving cells, but are also disadvantageous to downlink transmissions to an user equipment UE as will become apparent from the following.
In FIG. 37, a scenario with an eNodeB, a RRH and a user equipment UE is shown.
As explained above, the UE may be enabled for aggregation of downlink serving cells (component carriers) from different or same frequency band(s). In case the user equipment aggregates a first downlink serving cell provided by the RRH and a second downlink serving cell provided by the eNodeB, the user equipment receives downlink transmissions from the RRH and from the eNodeB subject to different propagation delays.
In other words, the user-equipment receives a downlink sub-frame of the serving cell provided by the eNodeB at a later point in time than the reception of the corresponding downlink sub-frame of the downlink serving cell provided by the RRH, wherein the term corresponding downlink sub-frames refers to sub-frames having a same sub-frame number. The different propagation delays are also illustrated in FIG. 37.
In particular, assuming the time of transmission of a downlink sub-frame of the serving cell provided by the eNodeB is at time t0, a user equipment receives this downlink sub-frame at time teNodeB. Further, in case of synchronous transmissions of corresponding downlink sub-frames by the eNodeB and the RRH, the time of transmission of a corresponding downlink sub-frame of the serving cell provided by the RRH is also at time t0 and the user equipment receives this downlink sub-frame at time tRRH. Even with a small propagation delay difference between the eNodeB and the RRH TPDeNB-RRH, the downlink serving cells of the eNodeB and the RRH are subject to different propagation delays resulting from its different locations and/or frequency selective channel effects.
The different propagation delays for downlink transmissions on the serving cells provided by the eNodeB and the RRH cause the user equipment to receive corresponding downlink sub-frames of different serving cells at different instances at time. Similarly, frequency selective routers (FSR) may also introduce different propagation delays for a user equipment, namely in case the user equipment aggregates downlink serving cells partly from the FSR and partly from an eNodeB.
Exemplary, in the scenario as shown in FIG. 37 the user equipment is positioned at a distance of 100 km from the eNodeB. For a distance of 100 km, the reception of a downlink sub-frame from the eNodeB is delayed with respect to the transmission time by 0.33 ms whereas the user equipment receives, due to its closer position to the RRH, a corresponding downlink sub-frame of the serving cell from the RRH earlier in time.
Different propagation delays are disadvantageous for the reception operation performed by the user equipment, since corresponding downlink sub-frames require to be simultaneously processed by the user equipment. In particular, corresponding downlink sub-frames may include interrelated information so that a user equipment can only process the information upon reception of corresponding sub-frames from all aggregated serving cells.
An obvious solution is to prescribe the user equipment to include a reception buffer for temporarily storing data for component carrier(s) respectively serving cell(s) during the time difference between the aggregated serving cell(s), e.g. in the above given example data of the delayed data from eNodeB needs to be buffered by the user equipment. Providing a large reception buffer however would result in various disadvantages.
Firstly, the realization of an user equipment with a reception buffer would not solve the problem of a delayed processing of downlink transmissions. In particular, since corresponding sub-frames of aggregated serving cells are required to be jointly processed by the user equipment, the latest reception time of one of plural corresponding sub-frames determines the start time of processing of the plural corresponding sub-frames such that all downlink transmissions of the aggregated serving cells are subject to a same delay.
Furthermore, the provision of a large reception buffer increases costs of the user equipment and is also disadvantageous to complexity of the implementation of the user equipment, e.g. in case of radii of up to 100 km, the required reception buffer size would be quite large.
Additionally, a reception operation of a implementation of a user equipment which would in turn increase complexity of the user equipment in terms of hardware, since corresponding downlink sub-frames on aggregated serving cells have to be identified in the reception buffer prior to joined processing thereof for decoding by the user equipment. In case of a small reception buffer only comprising corresponding sub-frames of aggregated serving cells, the user equipment would not be required to perform the additional identification of corresponding downlink sub-frames.